Bioelectrical stimulation electrodes may be judged according to many criteria, two of which are biocompatability with tissues in the body and low polarization impedance. Electrochemically, "polarization impedance" is a term which describes the inefficiency of an electrode in transferring energy to or from adjacent body tissues. Polarization impedance is a measure of the energy consumed in electrochemical reactions at the electrode-tissue interface. A low polarization impedance is important for stimulating the heart because a suprathreshold stimulating current can be applied to the heart which causes only slight potential changes. Therefore, electrochemical reactions with the body fluid are suppressed and the energy expenditure required for stimulating the heart is greatly reduced. Furthermore, a low polarization impedance is desirable for sensing intrinsic or natural bioelectrical signals, because for such intrinsic signals only a small measurement current flows and low polarization electrodes reduce the demands made on the input impedance of the sensing amplifiers. The practical result of using low polarization electrodes is to lessen the noise detected during the sensing of a bioelectrical signal. In addition, sophisticated bioelectrical stimulation devices may perform analysis of intrinsic bioelectrical signals and evoked potentials. Low polarization electrodes promote the capability to analyze these signals by abating problems arising from polarization artifact and noise signals, which otherwise could obscure desired physiological signals. Porous platinum electrode surfaces are especially desirable in this regard because of their high degree of biocompatability and their superior electrical properties.
U.S. Pat. No. 4,408,604, entitled "Porous Pacemaker Electrode Tip" granted on Oct. 11, 1983 to M. S. Hirshorn et al. discloses a porous pacemaker electrode tip comprising a concavo-convex electrode cap having a plurality of apertures therethrough and an electrode shaft having a supporting edge formed thereon to which the concave surface of the electrode is joined. The porous cardiac pacemaker electrode is formed by deforming a platinum plate onto a concavo-convex shaped cap member, thereby forming a plurality of selectively spaced apertures through the electrode cap member to make the electrode cap substantially porous.
U.S. Pat. No. 4,502,492, entitled "Low-Polarization Low-Threshold Electrode" granted to G. A. Bornzin on Mar. 5, 1985 teaches a cardiac pacing lead electrode having a low impedance by virtue of a platinum black coating and a novel geometry, involving circular grooves and tines to bring the electrode into contact with the endocardial wall in roughly a perpendicular configuration with respect to the grooves.
U.S. Pat. No. 4,542,752, entitled "Implantable Device having Porous Surface with Carbon Coating", issued Sep. 24, 1985 to A. DeHaan et al. discloses an electrode having the advantage of a low polarization impedance, which is accomplished by means of a porous carbon coating overlying a porous substrate or surface of an implantable device. The procedure for forming this porous carbon coating involves depositing a plasma coating by subjecting the substrate surface to a gaseous environment. This degrades the hydrocarbon in the substrate to form the porous carbon lattice structure.
U.S. Pat. No. 4,603,704, entitled "Electrode for Medical Applications", granted on Aug. 5, 1986 to K. Mund et al. describes a low polarization electrode comprising an electrically conductive carrier material and a porous layer in its active region, which is composed of a carbide, nitride or carbonite of one of the metals titanium, vanadium, zirconium, niobium, molybdenum, hafnium, tantalum or tungsten. The porous layers are applied to the substrate by means of physical vapor deposition.
U.S. Pat. No. 4,611,604, entitled "Bipolar Electrode for Medical Applications" issued to L Bodvidsson et al on Sep. 16, 1986 teaches an electrode with low polarization which is achieved by means of a toughened or porous material surface (e.g. a sintered metal alloy), underlying a layer of activated glassy carbon that has an extremely high double layer capacitance of up to 0.1 F/cm.sup.2. The best porous material surface is provided by a layer of a carbide, nitride or carbonitride of at least one of the metals titanium, vanadium, zirconium niobium, molybdenum, hafnium, tantalum or tungsten. The porous carbide, nitride or carbonium layers are situated upon an electrically conductive carrier material, for example platinum, titanium or a metal alloy such as Elgiloy.
U.S. Pat. No. 4,612,100, entitled "Method for the Fabrication of an Implantable Electrode", issued on Sep. 16, 1986 to M. Edeling et al. teaches a procedure for manufacturing an implantable stimulation electrode having a body portion with an outer surface, in which a layer of vitreous carbon is sputtered from a target of vitreous carbon onto at least a portion of the outer surface of the electrode body.
U.S. Pat. No. 4,649,937, entitled "Etched Grooved Electrode for Pacing Lead and Method for Making Same", issued Mar. 17, 1987 to A. DeHaan et al. provides a low impedance electrode having a plurality of grooves etched into the surface of the tip electrode to expand its surface area. The enlarged surface area lessens the polarization of the electrode while maintaining the overall diameter of the electrode so that the large surface area is within a small displaced surface area to ensure an electric current flow which is sufficient to cause muscle depolarization of the heart.
U.S. Pat. No. 4,679,572, entitled "Low Threshold Cardiac Pacing Electrodes", and U.S Pat. No. 4,762,136, entitled "Low Polarization Pacing Electrodes for Capture Verification", issued to R. G. Baker, Jr. on Jul. 14, 1987 and Aug. 9, 1988, respectively, disclose pacing electrodes which achieve low polarization by using an iridium oxide layer overlying the surface of the stimulating cathode.
Each of the aforementioned patents describes low polarization cardiac pacing leads, wherein the low polarization character of the leads is provided by either the application of special coatings to the pacing electrodes or by etching geometrical structures into an electrode. Either alternative increases the complexity of the process of manufacture for a pacing lead and raises the cost of a lead. What is desired is a method for reducing the polarization of a pacing lead that does not require application of a coating to an electrode and does not require painstaking application of solvents, etching solutions and photosensitive solutions which are required in the etching process.
It is known in the field of electrochemistry that the surface of metal electrodes immersed in aqueous solutions can be substantially modified by applying periodic electrical potentials of various waveforms. (For example, see A. Visintin et al., "Growth Modes of Platinum Overlayers Resulting from Square Wave Perturbing Potential Treatments of Different Symmetries", in J. Electrical Chem., Vol. 267, pp. 191-205, 1989.) The surface area of an electrode is electrochemically changed by varying the relative proportion of crystallographic faces through a process of electrochemical faceting and by producing different particular surface morphologies.
The work in the field of electrochemistry generally refers to a method for increasing the surface area of electrodes for usage in electrocatalysis. Heretofore, none of these electrochemical methods are known to have been applied to bioelectrical stimulation leads or electrodes. None of these electrochemical methods are known to have been applied to bioelectrical stimulation leads or electrodes for the purpose of increasing the surface area of pacing electrodes. Furthermore, none of these electrochemical methods are known to have been applied to bioelectrical stimulation leads or electrodes for the purpose of reducing the polarization of stimulating electrodes.
The object of the present invention is to provide a method for reducing the polarization of a stimulating lead without the necessity of applying a coating to the electrode. The present invention achieves low polarization by rearranging or roughening, by electrochemical means, the outer platinum layers of an electrode, resulting in a substantially increased microscopic electrode surface area.
What is also desired is a method for reducing the polarization of a stimulating lead which can reduce the polarization of a wide range of existing pacing leads. The method of the present invention may be applied to all existing noble metal-based pacemaker electrodes.
Furthermore, it is desired to reduce the polarization of leads in a simple, relatively fast, simply automated and easily implemented process which may take place on a production floor.